Far-infrared Laser Research Articles

Enhancement of far-infrared laser damage threshold in Ge-As-Se glass fiber

The application of As-Se chalcogenide glasses in high-power laser delivery is hindered by their low damage threshold due to their weak chemical bonds. To solve this, we introduced germanium elements into the As-Se glasses and optimized the composition to raise the glass transition temperature (Tg) and enhance the laser damage threshold (LDT). From the correlation among various parameters including Tg, LDT, and fiber loss, we concluded an optimized composition of Ge23As13.5Se63.5/Ge23As12.5Se64.5 glass. After effective purification and preform extrusion, the glasses were processed into high-quality step-index fiber (core diameter of 300 µm), which exhibits a minimum loss of 0.9 dB/m at 6.9 µm and is capable of withstanding an input CO2 laser power of 19.96 W at a wavelength of 10.6 µm, equal to a power density of 28.25 ± 0.02 kW/cm2 in air. This is nearly 2.3 times higher than that previously reported for Ge-As-Se fibers. This establishes a material foundation for achieving far-infrared high-energy laser systems using Ge-As-Se fibers in the future.

Quantum theory of the spin dynamics excited by ultrashort THz laser pulses in rare earth antiferromagnets. DyFeO3

A quantum theory of spin dynamics in the rare-earth orthoferrites excited by terahertz laser pulses is developed. The study demonstrates that dynamic magnetic configurations, triggered by a light pulse, exhibit stability even after the excitation source is ceased. The magnitude of post-excitation oscillations is linked to the ratio between the frequency of rare-earth ion excitations and the frequency of the external source. According to the analysis presented, dynamic response is significantly amplified when the system is exposed to ultrashort terahertz pulses. The physical characteristics of the oscillations emerging after the pulse are determined, and the factors governing their amplitude and phase are identified. The response signal is found to be dependent on the initial part of the pulse, specifically the half-period of the ultrashort light wave, while the subsequent part of the pulse contributes minimally to post-pulse magnetization dynamics. The findings highlight that in DyFeO3, terahertz dynamics primarily result from the influence of the magnetic field of the light, leading to excitations of electrons from the ground state to low-lying electronic levels of Dy3+ions. Additionally, the dynamic magnetoelectric effect excited by the electric field of the pulse is explored, revealing the emergence of odd magnetic modes.

Detecting Genetic Variation in Plants by Mapping Cell Water Dynamics With Terahertz Laser Feedback Interferometry

Detecting Genetic Variation in Plants by Mapping Cell Water Dynamics With Terahertz Laser Feedback Interferometry

High brightness terahertz quantum cascade laser with near-diffraction-limited Gaussian beam

High-power terahertz (THz) quantum cascade laser, as an emerging THz solid-state radiation source, is attracting attention for numerous applications including medicine, sensing, and communication. However, due to the sub-wavelength confinement of the waveguide structure, direct beam brightness upscaling with device area remains elusive due to several mode competition and external optical lens is normally used to enhance the THz beam brightness. Here, we propose a metallic THz photonic crystal resonator with a phase-engineered design for single mode surface emission over a broad area. The quantum cascade surface-emitting laser is capable of delivering an output peak power over 185 mW with a narrow beam divergence of 4.4° × 4.4° at 3.88 THz. A high beam brightness of 1.6 × 107 W sr−1m−2 with near-diffraction-limited M2 factors of 1.4 in both vertical and lateral directions is achieved from a large device area of 1.6 × 1.6 mm2 without using any optical lenses. The adjustable phase shift between the lattices enables a stable and high-intensity surface emission over a broad device area, which makes it an ideal light extractor for large-scale THz emitters. Our research paves the way to high brightness solid-state THz lasers and facilitates new applications in standoff THz imaging, detection, and diagnosis.

Fundamental bandwidth limits and shaping of frequency-modulated combs

Frequency-modulated (FM) combs based on active cavities like quantum cascade lasers have recently emerged as promising light sources in many spectral regions. Unlike passive modelocking, which generates amplitude modulation using the field’s amplitude, FM comb formation relies on the generation of phase modulation from the field’s phase. They can therefore be regarded as a phase-domain version of passive modelocking. However, while the ultimate scaling laws of passive modelocking have long been known—Haus showed in 1975 that pulses modelocked by a fast saturable absorber have a bandwidth proportional to effective gain bandwidth—the limits of FM combs have been much less clear. Here, we show that FM combs based on fast gain media are governed by the same fundamental limits, producing combs whose bandwidths are linear in the effective gain bandwidth. Not only do we show theoretically that the diffusive effect of gain curvature limits comb bandwidth, but we also show experimentally how this limit can be increased. By adding carefully designed resonant-loss structures that are evanescently coupled to the cavity of a terahertz laser, we reduce the curvature and increase the effective gain bandwidth of the laser, demonstrating bandwidth enhancement. Our results can better enable the creation of active chip-scale combs and be applied to a wide array of cavity geometries.

Theoretical Study of Molybdenum Separation from Molybdate Assisted by a Terahertz Laser.

Molybdenum (Mo) is a rare and important element extensively utilised in aerospace, radar communications, optoelectronic devices, and the military. This study proposes an environmentally friendly physical method based on photon-phonon resonance absorption for the separation of Mo from sodium molybdate (Na2MoO4). We examined the vibrational spectrum of Na2MoO4 using the CASTEP code, employing first-principles density functional theory. Through dynamic process analysis, we analysed the vibrational modes and assigned peaks corresponding to experimental infrared (IR) and Raman data. We focused on the vibrational modes associated with Mo and identified that the highest-intensity IR-active peak at 858 cm-1 corresponded to Mo-O bond asymmetric stretching. Therefore, we propose the use of a high-power terahertz laser at ~25 THz to facilitate the separation of Mo from Na2MoO4. Experimental investigations are expected in the future.

Floquet engineering of electronic states and optical absorption in laterally-coupled quantum rings under a magnetic field.

Tuning electronic and optical properties of low-dimensional quantum systems in a flexible way is of particular importance in designing semiconductor-based devices. Semiconductor quantum rings (QRs) are nanoscopic structures that have become promising systems for physical and technological applications due to their unique electronic and optical properties. Here, we explore the fundamental electronic and optical properties of laterally-coupled QRs by taking into account the combined effects of applied magnetic and non-resonant terahertz intense laser fields. The laser-dressed electronic states are solved using the Floquet theory of periodically driven quantum systems in high-frequency limits within the framework of effective mass approximation. We demonstrate that increasing the laser field parameter leads to reduced tunnel splitting in the energy spectrum and more pronounced Aharonov-Bohm oscillations for the ground state energy, due to the attenuation of the tunnel coupling. Furthermore, depending on the position of the avoided crossings in Aharonov-Bohm oscillations of laterally-coupled quantum rings, the evolution of the avoided crossings with increasing the laser field parameter can be elucidated as the attenuated tunnel coupling or the competition between the attenuated tunnel coupling and the strengthening anisotropy of the laser-dressed QR potential well. The intensity of the intraband optical transition of laterally-coupled QRs can be effectively tuned and reaches a larger value by manipulating the laser parameter and magnetic field. Optical Aharonov-Bohm oscillations in laterally-coupled QRs are still exhibited by changing laser field parameters. Our findings offer a novel approach to manipulate the electronic and optical performances as well as the Aharonov-Bohm oscillations based on the laterally-coupled QRs by using an intense terahertz laser field.

Quality improvement of BaGa4Se7 crystal by annealing in BaSe vapor atmosphere

Infrared nonlinear optical crystals are widely used in all-solid-state lasers as pivotal devices for laser frequency conversion. BaGa4Se7 (BGSe) crystal has attracted much attention due to its excellent optical properties. However, the presence of point defects in BGSe increases the absorption loss and reduces the laser-induced damage threshold, thus limiting its practical application in high-power mid- and far-infrared lasers. Therefore, it is crucial to reveal the types of defects and their formation mechanisms in BGSe crystal, which is conducive to the design of rational strategies to improve the crystal quality. In this work, the defect activation energies were calculated by photoluminescence (PL) spectroscopy and the existence of three types of defects, GaBa, VSe and VBa were verified in the as grown BGSe crystal. Therefore, a thermal annealing process under BaSe vapor was rationally designed to compensate for the loss of Ba and Se elements and to reduce the defect concentration in BGSe. In addition, the changes of defects in BGSe crystal were determined by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. All the experimental results confirm that the defect concentration in BGSe decreases greatly and the crystal quality is significantly improved after annealing in BaSe vapor.

Circular terahertz ratchets in a two-dimensionally modulated Dirac system

We report on the observation of the circular ratchet effect excited by terahertz laser radiation in a specially designed two-dimensional metamaterial consisting of a graphene monolayer deposited on a few-layer graphene gate patterned with an array of triangular antidots. We show that a periodically driven Dirac fermion system with spatial asymmetry converts the a.c. power into a d.c. current, whose direction reverses when the radiation helicity is switched. The circular ratchet effect is demonstrated for room temperature and a radiation frequency of 2.54 THz. It is shown that the ratchet current magnitude can be controllably tuned by the patterned and uniform back gate voltages. The results are analyzed in the light of the developed microscopic theory considering electronic and plasmonic mechanisms of the ratchet current formation. Published by the American Physical Society 2024

Properties of focused combined modes of terahertz laser

Physical features of spatial-energy characteristics of linearly polarized THz laser beams at sharp and moderate focusing in free space are analyzed both theoretically and experimentally. The Rayleigh–Sommerfeld vector theory is used to model propagation of laser beams excited by quasi-optical waveguide resonator modes in free space. Well-known methods of measuring spatial-energy characteristics of laser beams in the THz range are used in the experimental study. It is shown that the intensity of the total electric field of both combined TE0n + EH2n and EН–1n + EH3n modes (n = 1, 2, 3) in the focal region is defined by all three components and has a dip on the axis for both focusing types. The central maxima of the field of these modes become significantly shifted from the geometric foci of the lenses used as the order n of these modes increases. The EН–11 + EH31 mode has the smallest diameter (2.94λ) of the focal spot in the maximum intensity region at sharp focusing. The TE01 + EH21 mode has the smallest diameter (13.65λ) of the focal spot at moderate focusing.

Terahertz emission from diamond nitrogen-vacancy centers.

Coherent light sources emitting in the terahertz range are highly sought after for fundamental research and applications. Terahertz lasers rely on achieving population inversion. We demonstrate the generation of terahertz radiation using nitrogen-vacancy centers in a diamond single crystal. Population inversion is achieved through the Zeeman splitting of the S = 1 state in 15 tesla, resulting in a splitting of 0.42 terahertz, where the middle Sz = 0 sublevel is selectively pumped by visible light. To detect the terahertz radiation, we use a phase-sensitive terahertz setup, optimized for electron spin resonance (ESR) measurements. We determine the spin-lattice relaxation time up to 15 tesla using the light-induced ESR measurement, which shows the dominance of phonon-mediated relaxation and the high efficacy of the population inversion. The terahertz radiation is tunable by the magnetic field, thus these findings may lead to the next generation of tunable coherent terahertz sources.

Spectroscopy and efficient dual-wavelength laser performances of a Nd:GYSAG crystal.

We reported on the spectral properties and dual-wavelength laser performances of a novel, to the best of our knowledge, Nd:Gd1.8Y1.2ScAl4O12 (Nd:GYSAG) crystal for the first time. The absorption spectra, emission spectra, and fluorescence lifetime were systematically investigated. Further, a continuous-wavelength (CW) laser output power up to 5.02 W was obtained under an absorbed pump power of 9.45 W with slope and optical-to-optical efficiencies of 59.4% and 53.1%, respectively, at 1061.2 and 1063.2 nm. A stable passively Q-switched (PQS) laser employing Cr:YAG as a saturable absorber (SA) was realized. The maximum average output power of 0.756 W with a slope of near 34.4% was obtained with the pulse width, pulse energy, and peak power of 14.0 ns, 128.1 µJ, and 9.15 kW, respectively. The results indicate that the Nd:GYSAG crystal is an excellent laser medium for generating a high-efficiency dual-wavelength laser and has potential in terahertz (THz) laser generation.

High continuous-wave power surface emitting terahertz lasers integrated with multimode interferometer

Distributed feedback (DFB) surface emitting semiconductor laser realizes power extraction by periodic photonic heterostructure. A direct method to increase the output power is scaling up the area of active region, however, the length of the device is restricted by the coupling strength of the DFB grating. Herein we develop a high-power single-mode surface emitting scheme by monolithically integrating second-order DFB terahertz quantum cascade laser and a multimode interferometer, which can significantly enhance the optical power by increasing photon density and altering field distribution in the grating region. Application of this concept leads to high continuous-wave (CW) surface emitting power of 55 mW, with single-lobed far-field pattern and single-mode operation at about 3.9 THz, which is among the best reported results. Such an integrated scheme can be combined with any one-dimensional photonic heterostructure, which has great potential in further improving the pulsed and CW output power of single-mode semiconductor lasers.

Design of Far-Infrared High-Efficiency Polarization-Independent Retroreflective Metasurfaces.

Retroreflective gratings serve as fundamental optical elements in nanophotonics, with polarization-independent diffraction efficiency being one of the critical parameters for assessing their performance. In the far-infrared spectral range, traditional retroreflective gratings typically refer to metal echelette gratings, but their diffraction efficiency cannot approach 100% due to metal absorption. In the visible and near-infrared spectral ranges, metal echelette gratings have gradually been replaced by all-dielectric metasurfaces because dielectric materials exhibit negligible absorption at specific wavelengths. However, there is still a lack of relevant research in the far-infrared range, mainly due to the weak control capability of the existing devices over the polarization-independent phase. Here, we propose a kind of all-dielectric retroreflective metasurface composed of asymmetric pillars and freely tunable aperiodic multilayer films. The pillar structure can achieve polarization insensitivity, and the insufficient modulation capability of the dielectric materials can be compensated for by aperiodic Ge/ZnS films. The designed metasurface achieves the diffraction efficiency by RCWA, with the maximum larger than 99% and the overall reaching 95% (9.3-9.6 µm). We have provided detailed explanations of the design methodology and fabrication process. Our work lays the groundwork for further exploration and application of far-infrared lasers.

Helicity-Sensitive Terahertz Detection in Monolayer 1T'-WTe2.

Valleytronics, identified as electronic properties of the energy band extrema in momentum space, has been intensively revived following the emergence of two-dimensional transition metal dichalcogenides (TMDCs) as their valley information can be controlled and probed through the spin angular momentum of light. Previous optical investigations of valleytronics have been limited to the visible/near-infrared spectral regime through which the carriers of most TMDCs can be excited. Monolayer 1T'-WTe2 with broken time-reversal symmetry provides a fertile platform to study the long-wavelength photonic properties in different valleys. Here, we employed a circularly polarized terahertz (THz) laser to selectively excite the valley of monolayer 1T'-WTe2 and demonstrate that the helicity-dependent photoresponse is generated via the photogalvanic effect (PGE). We also observed that the photocurrent is controlled by circular polarization and the external electric field. Because of the tunable Berry curvature dipole derived from the nontrivial wave functions near the inverted gap edge in monolayer WTe2, the bandgap can be tuned efficiently. Our results provide a versatile venue for controlling, detecting, and processing valleytronics and applications in on-chip THz imaging and quantum information processing.

Characterization of RF System for MIR/THz Free Electron Lasers at Chiang Mai University

The establishment of the mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. The facility utilizes an S-band radio-frequency (RF) gun and a linear accelerator (linac) to generate and accelerate electron bunches. These electron bunches are accelerated in the RF gun and the linac using RF pulses with a frequency of 2856 MHz. Measuring the RF properties becomes essential, as the RF pulse information can be utilized to estimate the electron beam properties. To achieve the measurement results, we employed an RF measurement system comprising directional couplers, coaxial cables, attenuators, a crystal detector, and an oscilloscope. Prior to conducting measurements, the crystal detector and RF equipment were calibrated and characterized to ensure precise and reliable results. The electron beam energy estimation using the measured RF power was compared with the measured beam energies. The gun and the linac were operated with an absorbed RF power of 1.52 MW and an input power of 1.92 MW, respectively. The estimated electron beam energies were found to be 2.18 MeV and 15.0 MeV, respectively, closely aligning with the measured beam energies of 2.1 MeV and 14.0 MeV after the gun and linac acceleration. These consistent energy values support the reliability of our RF power measurement system and procedure.

The danger of infrared laser radiation to the organ of vision

Introduction. The article tells about the widespread use of infrared lasers in the fields of human activity. The constantly increasing power of laser installations leads to an increased risk to the health of large groups of workers. Current hygiene standards do not provide adequate eye protection from the harmful effects of this factor.
 The study aims are a scientific justification for improving the maximum permissible level (MPL) of infrared laser radiation.
 Materials and methods. To generalize and systematize the results of experimental studies and mathematical modeling to determine the threshold values of the harmful effects of infrared laser radiation on eyeball tissue, the authors used analytical methods.
 Results. Far infrared laser radiation can pose a danger not only to the cornea, but also to the retina. One of the important indicators affecting the depth of penetration of laser radiation into the structures of the eye and the severity of lesions is the absorption coefficient of the medium. Based on mathematical modeling of the thermochemical destruction of eye tissues, the researchers determined the threshold values of the harmful effects of far-infrared laser radiation, confirmed by the results of experimental studies. The scientists justified the need to introduce a higher coefficient of hygienic reserve in determining the maximum permissible level and recommended new proposals for the hygienic regulation of far-infrared laser radiation.
 Conclusion. The introduction of new maximum permissible levels will provide more reliable eye protection from far infrared laser radiation.
 Ethics. This study did not require the conclusion of the Ethics committee.

Forward modelling of the Cotton-Mouton effect polarimetry on EAST tokamak

Measurement of plasma electron density by far-infrared laser polarimetry has become a routine and indispensable tool in magnetic confinement fusion research. This article presents the design of a Cotton-Mouton polarimeter interferometer, which provides a reliable density measurement without fringe jumps. Cotton-Mouton effect on Experimental Advanced Superconducting Tokamak (EAST) is studied by Stokes equation with three parameters . It demonstrates that under the condition of a small Cotton-Mouton effect, parameter contains information about Cotton-Mouton effect which is proportional to the line-integrated density. For a typical EAST plasma, the magnitude of Cotton-Mouton effects is less than 2 for laser wavelength of 432 . Refractive effect due to density gradient is calculated to be negligible. Time modulation of Stokes parameters ( , ) provides heterodyne measurement. Due to the instabilities arising from laser oscillation and beam refraction in plasmas, it is necessary for the system to be insensitive to variations in the amplitude of the detection signal. Furthermore, it is shown that non-equal amplitude of X-mode and O-mode within a certain range only affects the DC offset of Stokes parameters ( , ) but does not greatly influence the phase measurements of Cotton-Mouton effects.

Theoretical Study of Efficient Photon-Phonon Resonance Absorption in the Tungsten-Related Vibrational Mode of Scheelite.

Tungsten (W) is an extremely rare and vital metal extensively used in metallurgy, the chemical industry, optoelectronic devices, and machinery manufacturing. In this work, an environmentally friendly and efficient physical method based on photon-phonon resonance absorption (PPRA) is proposed for separating W from scheelite. We calculated the vibrational spectrum of calcium tungstate (CaWO4) and assigned the infrared (IR) absorption and Raman scattering peaks through a dynamic analysis of the normal modes. We focused on the strong IR absorption peaks related to W and identified three high-intensity IR-active modes at around 830 cm-1, corresponding to the stretching of the W-O bonds. Therefore, we propose the use of high-power terahertz (∼25 THz) laser radiation to facilitate W extraction from compounds, leveraging the high efficiency of PPRA. Experimental testing is required to determine the precise absorption frequency under industrial production conditions.

Electron density measurement by the three boundary channels of HCOOH laser interferometer on the HL-3 tokamak

Far-infrared (FIR) interferometer is widely used to measure the electron density in the magnetically confined fusion plasma devices. A new FIR laser interferometer with a total of 13 channels (8 horizontal channels and 5 oblique channels) is under development on the HL-3 tokamak by using the formic-acid laser (HCOOH, f = 694 GHz). In order to investigate the boundary electron density activity during the divertor discharge, three horizontal interferometry channels located at Z = −97, −76, 76.5 cm have been successfully developed on HL-3 in 2023, and put into operation in recent experimental campaign, with a time resolution of < 1.0 μs and line-integrated electron density resolution of ~ 7.0 × 1016 m−2. This paper mainly focuses on the optical design of the three-channel interferometry system, as well as optical elements and recent experimental result on HL-3.